1. Field of the Invention
The present invention relates to a liquid crystal display device used for a computer, a word processor, etc. More specifically, the present invention relates to a liquid crystal display device with an improved display contrast and improved gray scale viewing angle dependence characteristics.
2. Description of the Related Art
As a liquid crystal display mode having outstanding viewing angle characteristics, a method using a liquid crystal cell having a homeotropic alignment film and an N-type nematic liquid crystal is known (Japanese Laid-open Publication No. 7-281176). According to this conventional method, as shown in FIG. 14A, a liquid crystal cell has a structure in which a pair of substrates 7 and 11 are provided so as to interpose a liquid crystal layer 9 made of an N-type nematic liquid crystal. Homeotropic alignment films 8 and 10 are formed on the substrates 7 and 11, respectively, on a liquid crystal layer 9 side, and phase difference compensation films 6 and 12 are provided on both sides of the liquid crystal cell. Alternatively, the phase difference compensation film may be provided only on one side of the liquid crystal cell.
Referring to FIG. 14B, the phase difference compensation films 6 and 12 will be described. Assuming that three principal refractive indices are na, nb, and nc in an orthogonal coordinate system, na=nb greater than nc, the direction of the principal refractive index na and the direction of the principal refractive index nb are within a plane substantially parallel to the surface of the substrate of the display device, and the direction of the principal refractive index nc is substantially vertical to the surface of the substrate.
Thus, in this liquid crystal display device, a black state can be obtained in every viewing angle direction while no voltage is being applied (i.e., when liquid crystal molecules are aligned substantially vertically to the surfaces of the substrates). In this state, the phase difference compensation film serves to correct the retardation of light output from the liquid crystal layer in any oblique direction.
In the above-mentioned liquid crystal display device, a satisfactory black display is obtained irrespective of an observation direction while no voltage is being applied, and as a result, viewing angle characteristics with an outstanding contrast ratio can be obtained. However, during a gray-scale display, gray scale characteristics are inverted at a predetermined viewing angle (e.g., at a viewing angle of about 10xc2x0 to about 50xc2x0) due to the retardation of the liquid crystal cell and the above-mentioned retardation correction characteristics of the phase difference compensation film.
A liquid crystal display device of the present invention includes: a liquid crystal cell including homeotropic alignment films which are provided on substrates interposing an N-type nematic liquid crystal layer on the liquid crystal layer side; a pair of polarizing plates provided so as to interpose the liquid crystal cell; and a phase difference compensation element provided between at least one of the pair of polarizing plates and the liquid crystal cell, wherein one principal axis of three principal axes of an index ellipsoid of the phase difference compensation element is inclined from a normal to a surface of the liquid crystal cell, and a refractive index along the one principal axis is smaller than a refractive index along the other principal axes.
In one embodiment of the present invention, the phase difference compensation element includes one phase difference compensation film.
In another embodiment of the present invention, the phase difference compensation element includes a plurality of phase difference compensation films having different optic axis polar angles, and an aggregate optic axis of the layered phase difference compensation films is inclined from the normal to the surface of the phase difference compensation element.
In another embodiment of the present invention, the phase difference compensation element is composed of layered phase difference compensation films including a plurality of phase difference compensation films with different optic axis polar angles and different azimuths, and the optic axes of the entire layered phase difference compensation films are inclined from the normal to the surface of the phase difference compensation element.
In another embodiment of the present invention, the phase difference compensation element is composed of one phase difference compensation film in which an optic axis polar angle is continuously changed, and an aggregate optic axis of the phase difference compensation film is inclined from the normal to the surface of the phase difference compensation element. Furthermore, the phase difference compensation element is composed of one phase difference compensation film in which an optic axis azimuth is continuously changed or one phase difference compensation film in which an optic axis polar angle and an azimuth are continuously changed, and an aggregate optic axis of the phase difference compensation film is inclined from the normal to the surface of the phase difference compensation element.
In another embodiment, in the phase difference compensation element, when a plurality of principal refractive indices of an index ellipsoid are na, nb, and nc, a relationship na=nb greater than nc is satisfied, at least one of a direction of the principal refractive index na and a direction of the principal refractive index nb is within a plane substantially parallel to the surface of the phase difference compensation element, and a direction of the principal refractive index nc is inclined from the normal to the surface of the phase difference compensation element.
In another embodiment of the present invention, in the phase difference compensation element, when a plurality of the principal refractive indices of an index ellipsoid are na, nb, and nc, relationships na greater than nc and nb greater than nc are satisfied, and a direction of the principal refractive index nc is inclined from the normal to the surface of the phase difference compensation element.
In another embodiment of the present invention, the above-mentioned liquid crystal display device further includes a second phase difference compensation element, wherein the one principal axis is inclined from the normal to the surface of the liquid crystal cell whereby the phase difference compensation element has anisotropy of a refractive index within a plane parallel to the surface of the liquid crystal cell, and the second phase difference compensation element cancels the anisotropy of a refractive index of the phase difference compensation element.
In another embodiment of the present invention, one principal axis of three principal axes of an index ellipsoid of the second phase difference compensation element is within a plane parallel to the surface of the liquid crystal cell, and a refractive index along the one principal axis is larger or smaller than refractive indices along the other two principal axes.
In another embodiment of the present invention, the second phase difference compensation element includes at least one phase difference compensation film.
In another embodiment of the present invention, the above-mentioned liquid crystal display device further includes a third phase difference compensation element between at least one of the pair of polarizing plates and the liquid crystal cell, wherein two principal axes of thee principal axes of an index ellipsoid of the third phase difference compensation element is within a plane parallel to the surface of the liquid crystal cell, and a magnitude of refractive indices along the two principal axes is different from each other, and a direction of the principal axis having a larger refractive index of the two principal axes is orthogonal to a direction of an absorption axis of an adjacent polarizing plate.
In another embodiment of the present invention, the third phase difference compensation element includes at least one phase difference compensation film.
Thus, the invention described herein makes possible the advantage of providing a liquid crystal display device having a wide viewing angle range without a gray scale inversion phenomenon in the case of using a liquid crystal cell provided with a homeotropic alignment film and an N-type nematic liquid crystal.